Zinc is one of the many trace minerals required by the body, but the fact that it's required in relatively small amounts should not be allowed to obscure its significance. It's required by the body for the production of more than 200 essential enzymes, one of the most crucial of these being superoxide dismutase, a vital anti-oxidant.

The particular importance of superoxide dismutase lies in its role in mopping up the superoxide free radicals which are released upon the metabolism of oxygen within cells, literally with every breath we take. Superoxide free radicals react with other by-products of energy production to form hydroxyl, the most damaging free radical of all, and to attack the mitochondria of the cells themselves.

Since it is the mitochondria which ultimately produce the energy for all the body's vital reactions, mitochondrial damage due to free radicals can only mean the production of less and less energy and the gradual deterioration and degeneration of the entire organism.

As well as being vital for the body's manufacture of anti-oxidants, zinc also helps the immune system to regulate the process by which defective or worn out cells are killed off by the body before they have chance to multiply themselves and perhaps ultimately become cancerous. These functions alone would make it one of the key micronutrients which are vital to the body's long-term health.

But there's much more to zinc than this, because it's now known to be necessary for the production of the white blood cells which are one of the body's first lines of defence against infection, and for the proper functioning of the thymus gland which is heavily involved in the regulation of the immune system. So zinc is crucial not just in staving off long-term degeneration, but in fighting off acute infections.

And this well documented value as an immune system booster has given zinc great popularity as a cold and flu remedy, although the effectiveness of the many lozenges sold for this purpose once illness is established remains open to question. But the importance of a healthy immune system in preventing infection in the first place is not in doubt. And it's here that an adequate supply of zinc is essential.

Although the typical requirement for zinc is only 15 mg a day, it is surprisingly easy to fall below this. Indeed some estimates suggest that typical dietary intakes may be as low as 9 mg for women and 13 mg for men, and even this may be poorly absorbed so some nutritionists recommend aiming for an intake of around 25 mg to ensure an adequate supply.

It also needs to be remembered that certain groups of people with depressed immune systems, such as HIV sufferers, diabetics and alcoholics may have an increased requirement for zinc. But by far the largest such group is comprised of the over 65s. And it should be realised that the consequence of a zinc deficiency may be much more serious for these older individuals, given that their immune systems are likely in any case to be less effective than those of healthy young adults.

So the conclusion appears to be straightforward: ensuring that the diet is adequate in zinc is vital to the efficiency of the immune system, and therefore to long-term health and well being. But as always with the human body, the full story isn't quite so simple. The body functions holistically, and an excess of any one nutrient may well lead to a deficiency in another. Taking in too much iron, for example, may in itself lead to a deficiency in zinc. But in the case of an excess of zinc (defined by the US Food and Nutrition Board as more than 40 mg a day), the associated deficiency is of copper.

Deficiency in copper is rarely severe enough to produce clinical symptoms, but this should not obscure the important functions of this trace mineral. As with zinc, adequate supplies of copper are vital for the production of anti-oxidant enzymes and energy within the body's cells. Sub-clinical deficiencies have also been implicated in cases of mild anaemia and depression of the immune system.

The best way to counter these risks is, of course, to consume a diet rich in both zinc and copper. Red meat, particularly beef, and shellfish are known as good animal sources, whilst nuts and beans offer a useful source for vegetarians. Whole grains are a useful additional plant source of copper. Plant sources of zinc, however, are less well absorbed by the body than those derived from animal sources, and so this is a nutrient which may require particular attention for those following a vegetarian diet.

But given the poverty of modern Western diets in both vitamins and minerals, supplementation may in any case be necessary to ensure the health of the immune system, particularly, as discussed above, for the over 65s. Various forms of specific zinc supplement are readily available, but it is probably wiser to ensure that any supplementation of zinc is taken in a properly balanced multi-mineral formulation also containing an appropriate amount of copper.

The eye is a delicate organ and the components that it contains, such as the lens, retina and macula are prone to free-radical attack. They can be protected with the super-powerful antioxidants like Vitamin C and Vitamin E.